The present invention relates generally to measuring characteristics of soil and, more particularly, to a method and apparatus for determining the salinity or salt content of soil.
The salinization of soil is generally caused over time by the transportation of small quantities of inorganic salts into the soil by irrigation water. As the irrigation water evaporates and is transpired, the salts are left behind and may accumulate to cause damage to plant life unless sufficient irrigation water is provided to leach the salt to below the root zone of plants occupying the soil.
High levels of irrigation unfortunately may not be a solution to soil salinity problems even if unlimited amounts of water are available. Upward capillary transport and evaporation of saline ground water causes a high salt concentration just above the water table. Over irrigation may raise the water table bringing the high salinity region into the root zone of plants occupying the soil. Thus, it can be seen that soil salinity measuring devices are needed to gauge the extent of salinity problems, to monitor changes in soil salinity level over time, and to provide a guide to optimizing crop yields and preserving or reclaiming agricultural lands which may be threatened by salinity problems.
The standard method of measuring soil salinity is to form a saturated paste by adding water to a soil sample and then extracting the soil pore water using vacuum filtration. This is described in detail in the USDA Handbook 60, 1954, "Diagnosis and Improvement of Saline and Alkali Soils." The salinity of the soil pore water extracted is gauged by its electrical conductivity. Saturation water content is used because it is repeatable and is the lowest practical water content for extraction purposes. Unfortunately, the extraction method for determining soil salinity is time consuming and is limited to performance in a laboratory.
As an alternative to the extraction of soil pore water from saturated pastes of soil samples, soil pore water may be collected by means of vacuum extractors which are placed in the field for in situ soil water sampling. Unfortunately, such in situ sampling is influenced by sampler in-take rate, plugging and sampler depth and size such that serious doubts remain about the representativeness of water samples collected by in situ extraction.
Another approach to in situ salinity monitoring has been buried porous salinity sensors which imbibe and come to diffusional equilibrium with the soil water. Unfortunately, such sensors have long response times which may be several days and are not accurate in dry soils.
Soil salinity can also be determined by measuring soil water electrical conductivity using four electrode or Wenner arrays and electrical measuring equipment. Unfortunately, the electrical conductivity of bulk soils is greatly effected by the soil type, as well as by salt and water contents.
While one or a combination of these known salinity measuring arrangements may be used for a given application, new alternatives for quickly and inexpensively measuring soil salinity are in demand and serve to advance the art of soil salinity measurement to facilitate soil studies and soil conservation.